For statistical and supervisory purposes it is known to provide voltage sensors which are periodically connectable to these test points and deliver information on the activity or state of the subscriber lines or other signal sources involved. Thus, for example, a known traffic monitor manufactured by the German firm of Siemens and Halske AG, marketed under the designation VGA 68, comprises a portable assembly of several insertion networks or loops each including a number of panels with one master relay and ten slave relays per panel, the master relays of the various panels being connected in cascade for sequential operation. Under the control of the master relay, the ten slave relays of a panel are also successively operated; with 12 armatures per slave relay, a panel is able to couple up to 120 test wires of respective telephone lines to a set of associated voltage sensors.
All these prior-art monitoring units are generally of limited applicability regarding the nature and/or the mode of operation of the signaling devices to be checked. In many instances, the voltage level varies significantly among different groups of wires so that threshold sensors with different reference voltages must be used for their testing. The aforedescribed traffic monitor VGA 68, in particular, includes means for emitting a special marking signal calling for the switching of sensors upon a changeover from one wire group to another.